The present invention relates to a circuit for sensing output distortion, in particular of final stages of audio devices.
As is known, an audio device can be compared to an operational amplifier which is fedback with a final stage such as the one shown schematically in FIG. 1. As can be seen, the final stage 3 can be represented by an audio amplifier 1 which drives a pair of transistors Q.sub.1, Q.sub.2 which are connected between the two power supply lines at +V.sub.CC and -V.sub.CC and define, between them, the output OUT of the audio device, to which the load, represented herein by a loudspeaker 2, is connected. The feedback circuit is constituted by a pair of resistors R.sub.1, R.sub.2 ; specifically, the resistor R.sub.1 is connected between the inverting input of the amplifier 1 and the ground and the resistor R.sub.2 is connected between said inverting input and the output OUT; the input signal V.sub.IN is applied to the non-inverting input of the amplifier.
In the final stage shown in FIG. 1, the maximum swing of the output, which is theoretically limited by the difference between the positive supply voltage and the negative one (2 V.sub.CC) is actually given by EQU .DELTA.V.sub.OUT =2V.sub.CC -V.sub.CEQ1,sat -V.sub.CEQ2,sat.
When the input signal V.sub.IN exceeds a certain dynamic range equal to ##EQU1## the output is no longer able to follow the dynamics of the input signal in a linear manner, and distortion occurs (the so-called "clipping" phenomenon). Since said output signal drives a loudspeaker, in order to be heard with a still acceptable fidelity its distortion during the peaks of the input signal must not exceed a certain percentage, e.g. 10%; beyond this value the sound in fact becomes unpleasant.
In order to solve this problem it is possible to sense the existing distortion and drive an appropriate attenuation circuit which is connected upstream of the final stage so as to limit the input signal thereof.
A solution for sensing the distortion is known for example from the U.S. patent application Ser. No. 07/202,846 filed on June 6, 1988 in the name of the same Applicant, wherein sensors are provided which control the state of saturation of the final transistors. Though it adequately solves the problem in some cases, this solution is in any case not free from disadvantages, since the information related to a 5-10% distortion, obtained by integrating the output signal from the clipping sensor, is not immediately available due to the integration time constant; the solution is applicable only in amplifiers with a particular output stage and is sufficiently accurate only if the output waveform of the amplifier is symmetrically distorted.
Other known solutions consider the input or output voltage signal and compare it with a reference voltage. A solution in which the input signal is controlled is shown by way of example in FIG. 2, wherein the final stage 3, which has a feedback network R.sub.1, R.sub.2, drives a load R.sub.L. The input voltage V.sub.IN is also supplied to the positive input of a comparator 4; a fixed reference voltage is provided on the negative input of said comparator, which generates an error voltage V.sub.O which is then used to drive an attenuator. These known solutions, however, are not satisfactory, since they do not take into account the power supply voltage and/or the maximum swing of the signal in output from the audio amplifier, which also depends on the type of load to which the latter is connected.